The subunit dissociation of heart mitochondrial malate dehydrogenase, and the complex between malate dehydrogenase asparate amino transferase, will be studied. The effects of various modifiers on the kinetics and equilibria of the subunit dissociation will be determined. Effects of the formation of a complex between malate dehydrogenase and aspartate amino transferase on the kinetic properties of each enzyme will be investigated. The physiological significance of these interactions will be evaluated.